The Benefits of Gear Honing

This Präwema Model HS205-D Double Spindle Gear Hone is connected to an automation system (courtesy of American Wera).

Economically, gear honing, has become an essential part in the production of high-speed transmissions. Gears that have been honed—compared to ground—offer excellent wear characteristics and are extremely quiet. But the benefits of this finishing technology don't end with high-speed transmissions. Gear Technology recently caught up with American Wera, the U.S. partner for Präwema honing machine technology, to discuss the latest honing advancements featured at EMO Hannover 2011 and the future of the process itself.

Scott Knoy, vice president of sales at American Wera Inc., believes gear honing can be a cost effective manner of fine finishing gears that assures a quiet gear set and the best resistance to micropitting. A honing machine and accompanying equipment (filter) is a less expensive system than a comparable gear grinding system and the tooling run each is comparable. "That being said, if you desire a cost competitive process, it is essential to tightly control your upstream processes as honing is effective on gears with a maximum stock removal of 75-80 microns per flank," Knoy says. "Manufacturers generally target a stock allowance of between 45-60 microns per flank. This range ensures a fast cycle time and good tool life in addition to the benefits of pitting resistance and quietness. If the stock allowance exceeds 70 microns per flank, the process will still provide all of the benefits except a short cycle time and acceptable tool life."

Gear honing is a universal technology, but is generally used in automotive, aerospace, truck and heavy equipment markets. It's a suitable method for any application where quiet, robust and reliable gearing is a requirement. "As hybrid and electric technology evolves in the automotive industry, gear noise is an issue. In these models the engine noise is greatly reduced and people will be able to hear any gear noise. OEMs' are aware of this and are increasing the use of noise eliminating gear honing in these applications. Additionally, applications such as heavy truck transmissions where robustness and durability is a must, honing is the preferred method of gear finishing. The pitting resistance provided by gear honing shows dramatic increases in the life of these truck gears," Knoy says.

In addition, the finishing operation offers other significant advantages that many people in the industry do not consider. "One, the gear honing process will not produce burning on the gear flanks as gear grinding can. As we know a burnt gear flank will result in pre-mature failure. Two, the gear honing process does not introduce bias (twist) into the part as gear grinding naturally does. However, if bias is wanted then the hone stone can be dressed to produce the desired bias. Three, gear honing is perfect for finishing gears with very little clearance due to shoulders or because of multiple gears on a shaft, areas where grinders cannot operate. We have processed gears with as little as 1.8 mm clearance in the past."

American Wera, Inc. has two current honing machines available: the Präwema HS205 and the HS205-D (double work spindle). "As you know our customer base is interested in making high quality gears at an accelerated rate at a good cost per part. These demands propelled us to develop the double spindle machine (HS205-D). This model has allowed us to process gears in cycle times as fast as 15 seconds, floor to floor, including roll checking (pre and post) and pro-rated dressing time. As these new models delivered we were pleased that our customer base reported that the technology worked as promised, and that the durability and reliability of the machine matched that of the widely tested and accepted HS205," Knoy says.

Since the advent of "power" honing machines most advances have been made through abrasive technology. This is related to the fact that the direct drives spindles can operate at much greater speeds than the current abrasive technology can handle. "Until the abrasive technology catches up with the machine technology we have focused in areas that increase the Overall Equipment Effectiveness (OEE) of our product line," Knoy adds.

The Präwema Model HS205 (single spindle gear hone) is one of two machines current available (courtesy of American Wera).

At EMO Hannover, Präwema debuted three new technologies: on board measurement using a unit and software similar to that used on standard gear measuring equipment, a new system for profiling "blank" hone stones and VarioDressing.

"The onboard measuring system is aimed at decreasing the downtime associated with 'first' piece checks and routine quality inspections," Knoy says. "It provides the operator with a chart similar to what is seen from gear measurement machinery. A good chart will allow the operator to continue honing parts while they confirm the results with the official measuring machines located elsewhere in the plant, allowing production to continue. Conversely a poor chart will signal that there is an issue to be resolved."

The profiling technology introduced at EMO allows manufacturers to bring in "blank" hone stones and profile any form they would like into the stone in approximately three minutes. "The cost savings of roughly 50 Euros per hone stone are realized by bringing them in as blank stones," Knoy adds. "Additional benefits to the user include inventory reduction as a blank can be used for any gear being processed and at the end of life the stone can now be re-profiled using a different number of teeth to process the same gear or re-profiled for use with a completely different gear."

Präwema VarioDressing is a universal dresser for testing new part designs and initial stages of production with newly developed gears. "Instead of buying new tools and waiting for them to be manufactured you can make geometry changes using the machines axes; saving time and money. Once the part geometry is established the form can be integrated into a standard dressing tool for production."

While Knoy believes that current honing technology is aimed at increasing the tool life of the abrasives, fine-tuning/reducing the dressing cycle and increasing the operational efficiency of the machine, the technology is always adapting to the application needs.

"In the near future we see that increases in abrasive technology will permit the hones to increase their throughput. To that end we are working with bearing manufacturers to increase the quality and strength of the ceramic bearings to handle the increased speeds providing a continued stable process and guaranteeing an acceptable spindle life," Knoy says.

Knoy notes that honing is a growing business and sales have climbed year after year since the company began manufacturing hones in 1999.

"At this point gear hone manufacturers compete as much with the grinding process as they do between themselves. This is due to the noise and durability characteristics that honing process provides versus the forgiveness provided by the grinding process," Knoy says. "Each manufacturer has to decide which elements are most important to them and benefit them most."

More Honing Options

Fässler: The Fässler HMX-400 Gear Honing Machine was brought on the market to finish high quality gears directly after hobbing and heat-treating very rapidly and economically. The costs per part are close to green shaving—but with the quality advantages of a hard finishing process. Honed gears produce less noise and have a longer life than other gears due to their typical surface structure. The structure of the surface of a honed gear, which resembles a fish skeleton, facilitates the formation of a lubrication film surface from the tip of the flanks to the pitch diameter and thereby positively influences the noise behavior in the gearbox. The special process kinematics cause this surface structure by which the honing tooling makes rolling contact on opposed axes of the workpiece. The resulting speed division on the gear flanks results in having a cutting speed component in the axial direction so that the cutting grain comes into contact with the entire flank.
For more information, visit www.faessler-usa.com.

Sunnen: Thanks to recent advances in honing technology, Sunnen Products Company can help gear manufacturers achieve greater precision and increase productivity 60 percent or more over previous honing machines or I.D. grinding.
The advanced technology is incorporated into a new line of honing machines, including Sunnen's ML-5000 automatic honing system. The computer controlled ML-5000 can hold bore size tolerances to within 5 microns, with virtually any surface finish desired, to meet stringent gear specifications. The ML-5000 is suitable for precision medium-to-high production of gears with bores from 1.5-101.6 mm. A new cycle time control feature automatically controls the feed pressure to optimize abrasive performance, assuring increased productivity and consistency. Greater honing speed, consistency and precision all add up to cost savings that can mean payback in just months. The ML-5000 can be automated for even greater productivity and reduction of labor costs. The improvements in honing machines have been accompanied by new technology in honing tools. Sunnen's TurboHone (MMT) tools typically cut twice as fast as standard honing tools, while providing the bore size and finish required by today's gear manufacturers. Costs can be reduced as much as 50 percent in many applications when TurboHone MMT tools are used. In addition to the ML-5000 honing system, Sunnen offers a wide variety of single or multi-spindle honing machines in either vertical or horizontal configurations to fit virtually any gear bore sizing and finishing application.
For more information, visit www.sunnen.com.

Sicmat: Sicmat entered the gear honing field with the Grono 250. Developed through the Gear Related External Teeth Tool Honing Application (GRETHA) project in cooperation with the Faculty of Engineering at Turin Polytechnic
University, this machine represents the latest in post-hardening fine finishing technology. The mission of the Grono 250 is to keep the advantages of Raso range in honing: simple construction, user practicality and a lower initial investment with lower costs in the production process. Grono 250 is an innovative honing machine that transfers the advantages of shaving allowing restricted investments and management costs to the post heat treatment finishing process. The Grono 250 features a honing wheel with external toothing design, easy accessibility for maintenance and operations, vertical workpiece axis for an easier link to automation, a main machining axes operated by direct-drive motors, workpiece/tool synchronization, honing wheel axis and workpiece axis operated by electro-spindles and an integrated pre-process measuring station. Star SU represents Sicmat in the United States, Canada and Mexico.
For more information, visit www.star-su.com.

Gleason: Gleason's 150SPH Power Honing Machine uses the latest Gleason patented Spheric Honing process to finish hardened spur and helical gears up to 150 mm in diameter in as little as half the time required by existing machines. The 150SPH is well-suited for production environments requiring higher volume finishing of high precision gears with low noise characteristics, of the type increasingly found in many of today's transmission applications. Equipped with a high speed direct-drive honing head, the 150SPH is capable of honing at speeds almost twice as fast as commonly used in practice today. It also employs Gleason's patented Spheric Honing process, a combination of Gleason proprietary software and powerful Siemens 840D CNC that enables the user to produce quieter gears faster by improving tooth geometry, reducing concentricity and indexing errors, and optimizing overall tooth flank contact characteristics.
The Spheric Honing process, first introduced for use by Gleason's popular ZH 125 and ZH 250 honing machines, offers many additional benefits to the user. For example, tooth trace and crowning modifications are generated by machine movements only, so no change is necessary to the dressing master when modifications are required or when changing over from one workpiece type to another, thus reducing tooling costs and non-productive changeover time. When dressing is required, a patented process is used that can extend by as much as four times the useful life of the honing tool, resulting in significantly lower tooling costs per workpiece.
In addition, the process enhances the compressive stress in the surface of the tooth flanks to help prevent early wear and extend gear durability and useful life. The use of the latest Siemens 840D CNC and Gleason Windows-based Intelligent Dialogue software also greatly simplifies setup and operation. For example, when flank form corrections are required after measurement, they can be made easily, and automatically by the machine, helping even less experienced operators to produce high quality parts more productively.
For more information, visit www.gleason.com.